One proposed battery device determines a voltage adjustment condition of plural cells, which are arranged in series to construct a battery assembly, according to the charge levels or the states of charge SOC of the respective cells, and actually adjusts the voltages of the respective cells in the battery assembly on the basis of the determined voltage adjustment condition (see, for example, Japanese Patent Laid-Open Gazette No. 2004-31012). Such voltage adjustment equalizes the voltages of the respective cells in the battery assembly.